The present invention relates to an optically powered signal transmission apparatus which communicates bidirectionally between two stations, with electric power supplied from one to the other through a light beam.
Many optical transmission systems are known which transmit data through an optical fiber cable transmission network between two or more stations. Such systems offer enhanced data security, immunity to electromagnetic noise (EMI and RFI), and excellent explosion-proof properties. This latter feature makes such systems ideally suited for operating in potentially explosive atmospheres.
FIG. 7 is a block diagram showing a conventional optical signal transmission apparatus according to the prior art. A control station 10 controls two field stations 20 and 30. Field station 20 typically contains transducers and works as a sensor which measures temperature, pressure, etc. or works as an actuator which adjusts temperature, pressure, etc. Control station 10 includes an optical transceiver circuit 13a for use with field station 20. Control station 10 also includes an optical transceiver circuit 13b for use with field station 30. Field station 30 is identical to field station 20, and optical transceiver circuit 13b is identical to optical transceiver circuit 13a. The descriptions of field station 30 and optical transceiver circuit 13b are therefore omitted.
Optical transceiver circuit 13a includes a light source 131 for producing a light beam. The light beam passes through an optical light splitter-coupler 133' and an optical fiber 41 to field station 20. In field station 20, the light beam enters a light splitter-coupler 23' where it is reflected to a light receiver 221'. Light receiver 221' converts the optical signal to an electrical signal which is sent to a control circuit 21. Control circuit 21 is an interface with any type of transducer (not shown) or sensor (not shown) that are in field station 20. Control circuit 21 detects control data in the electric signal. Control circuit 21 controls a light source 222' for producing an outgoing light beam which passes through light splitter-coupler 23' and optical fiber 41. The light beam is reflected by optical light splitter-coupler 133' to a light receiver 132. Light receiver 132 converts the optical signal to an electrical signal which is sent to a control circuit 11. Control circuit 11 detects data produced by field station 20. Optical fiber 41 is connected to control station 10 by a connector 51a and to field station 20 by a connector 51b. A battery 27 powers field station 20.
Field stations 20, 30 are assumed to be placed in adverse environments, such as, for example, in dangerous areas in an oil refinery or chemical factory, in which a potentially explosive atmosphere exists. Because of this, battery 27 must be explosion-proof or meet intrinsic safety conditions for avoiding high voltages.
In addition, battery 27 must be replaced on a regular basis. Care must be taken in designing and locating field station 20 to facilitate battery replacement. The conventional optical signal transmission system is therefore subject to high material and labor costs for replacing batteries.
For these reasons, optically powered signal transmission systems have been developed which eliminate the need for a battery. Solar cells are used to convert light, transmitted through a power optical fiber, to electric power to power the field station. The return signal from the field station is normally carried over a separate signal optical fiber. This requires two optical fibers; the power optical fiber and the signal optical fiber.
Sensor systems are being developed which use one optical fiber for both power and signal. A power light beam is sent from a control station to a remote sensor over an optical fiber where part of the light is converted to electricity to power the sensor. The remainder of the light beam, after being modulated with a digital signal produced by the sensor output, returns to the control station via the same optical fiber. The amount of light converted to electric power is quite limited. In addition, optical fiber lengths are limited due to the necessity for the optical signal to return to the control station without undue attenuation. Another drawback to such systems is that the signal communication is one-way, from the sensor to the control station. Bidirectional communications requires either a battery at the field station or two cables, one for the power and one for the signal.